1. Field of the Invention
The present invention relates to technology for suppressing ESD (also called electrostatic discharge damage) of a semiconductor integrated circuit chip mounted on an IC card. More particularly, the invention relates to technology that can be effectively adapted to memory cards such as multi-media cards.
2. Prior Art
A variety of memory cards have heretofore been provided for storing multi-media data in compact sizes having decreased weights. For example, there has been provided a multi-media card having a memory and a memory controller mounted on a card substrate, establishing an interface to a host unit using a small number of signals.
Giving priority to a small size and reduced weight, the memory card of this type has the connection terminals for connection to the host unit, the connection terminals being exposed on the card substrate, but has no particular mechanism such as cover for protecting the terminals. When the exposed terminals are touched at the time when the memory card is removed from the host unit, therefore, a semiconductor integrated circuit chip connected to the exposed terminals may be damaged. Usually, the semiconductor integrated circuit chip is provided with an input protection circuit for preventing the input circuit from the electrostatic damage. The input protection circuit is constituted by, for example, disposing, between an input terminal and a power source terminal, an element such as a diode whose connection state is reversed relative to the amplitude voltage of the input signal. It is, however, expected that the memory card is carried by itself and is frequently attached to, and detached from, the host unit. Thus, the present inventors have found the importance of reinforcing the prevention of the electrostatic damage.
Though the technical field is different from the above memory card, Japanese Patent Laid-Open No. 209379/1998 discloses technology for reinforcing the protection of input against the electrostatic damage. According to this technology, a metal wiring layer is formed maintaining a gap (discharge gap) in which static electricity could be discharged relative to an electrode layer on a semiconductor substrate so that when static electricity has entered into the electrode layer, the static electricity is discharged to the metal wiring layer, preventing the static electricity that has entered into the electrode layer from entering into the semiconductor element. Further, Japanese Patent Laid-Open No. 271937/1995 discloses a circuit employing a gate-source protection diode of a MOSFET that is externally attached to a semiconductor integrated circuit chip to prevent the electrostatic damage.
There has further been provided a varistor of semiconductor ceramics from the standpoint of protecting the circuitry from an overvoltage.